The present invention relates to protecting neodymium-boron-iron magnets and to protecting and bonding such magnets in assemblies, especially motor field assemblies.
Neodymium-boron-iron magnets have recently found various applications, for example, in electric motor field assemblies. They are, however, highly susceptible to corrosion and oxidation. The heavy corrosion observed with Nd-Fe-B alloys can proceed up to a complete pulverization of sintered or compacted bodies.
Oxidation protection of Nd-Fe-B magnets has included modification of the alloy elements and use of chloride free powders and various coatings including aluminum vapor deposit, titanium nickel, and epoxy. It is reported that most metallic coatings, including aluminum, copper, cadmium, nickel, and titanium nickel exhibited depletion from the surface after being kept, even in air, for a couple of years. Some epoxy coated magnets exhibited excellent corrosion resistance. Liquid epoxies, as opposed to conventional solid types, permit a rapid solubilization of the chromate molecules when chromate-rich primers are used. This causes corresponding blisters when topcoated and subsequent failure during hot water or humidity tests. When the sacrificial chromate is omitted, blistering does not occur; however, corrosion resistance is unacceptable. Other coating problems include permeability of the coating and chemical reaction between the magnet and the coating.
It has been found that coatings and processes suitable for steel are not necessarily acceptable for coating neodymium magnets. For example, most metal platings are sacrificial with respect to the base material but neodymium, due to its high activity level, will actually sacrifice itself to protect the plating.
It has been found that known electroplated coatings, including cadmium or nickel plating, do not provide adequate corrosion protection. Indeed, in some cases such plating processes actually contribute to corrosion problems.
A further problem is present when neodymium-boron-iron magnets are used in motor field assemblies since this magnetic material possesses a negative coefficient of thermal expansion in the plane perpendicular to the axis of magnetic orientation. Since motor assemblies must operate over a wide thermal range, the magnet can experience thermal expansion far different than the substrate, usually iron, to which it is attached. The effects of corrosion and thermal expansion have been found to cause neodymium-boron-iron magnets to break loose from the motor field assembly as the motor is temperature cycled. In such cases, the corroded magnets simply cannot withstand the large difference in thermal expansion between the magnetic material and the field assembly substrate. Since the corroded magnet surface layer can no longer support the large stress gradient at the bond interface, the magnet fractures along the bond surface and is lost within the motor. Naturally, this detracts from or inhibits motor performance.
Therefore, it is one object of the present invention to provide adequate corrosion protection for neodymium-boron-iron magnets and motor field assemblies. It is another object to prevent such magnets from breaking loose in motor field assemblies.
These and other highly desirable and unusual results are accomplished in accordance with the present invention in a reliable, cost-effective manner so that the full advantages of neodymiun-boron-iron magnets may be realized.
Objects and advantages of the invention are set forth in part herein and in part will be readily appreciated herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.